1. Field of the Invention
The present invention relates to a method for driving such display devices as a liquid crystal display device, thin film EL display device, and the like.
2. Description of the Prior Art
For a liquid crystal display device and the like, a frame thinning out system is well-known as one of driving systems for enabling such a display panel to display a gradationally toned image with several brightness levels on its screen.
The frame thinning out system turns on pixels only in a number of frames corresponding to a level of brightness to be displayed, out of all frames within an integration time period consisting of a plurality of frame time periods, so as to visually obtain a medium level of brightness in each integration time period.
Table 1 shows an example of conventional relation between each brightness level and frame time period for each pixel to be turned on in case of obtaining a gradational display by setting four frame time periods as one integration time period by means of the frame thinning out system. Further, each pixel is turned on in the order of frames shown in this table according to the brightness level to be displayed.
TABLE 1 ______________________________________ Frame number Gradation level 1 2 3 4 ______________________________________ 4/4 1 1 1 1 3/4 1 1 1 0 2/4 1 0 1 0 0/4 0 0 0 0 ______________________________________
According to the conventional driving method mentioned above, however, brightness of pixels varies every four frame time periods as shown in FIG. 1, in the case of displaying the 3/4 gradation level in Table 1, for example. In this case, a frame frequency of 80 Hz makes a brightness variation frequency of 20 Hz. Since this frequency is lower than the maximum frequency (30 Hz) in which a human being generally feels variation of brightness as flicker (hereinafter referred to as flicker), flicker is caused in the unit image area displaying the 3/4 gradation level. Since generally a common gradation level appears on the screen as a whole of a unit image area displayed by a group of plural pixels, the flicker is intensely felt on the screen.
In particular, since the number of frames in one integration time period is increased in proportion to increasing the number of gradation levels in the case of the frame thinning out system, the frequency of brightness variation becomes lower in proportion, to increase of the number of gradation levels. Thus, as a result, the more intensified flicker remarkably deteriorates quality of the display.